In the field of magnetic resonance (MR) imaging, and in particular for multi-transmit MR imaging applications, it is known to especially use two types of degenerate radio frequency (RF) coils as antennae for exciting nuclear spins within a subject of interest, usually a patient, and detecting signals from them. These antennae, birdcage coil and transverse electromagnetic (TEM) coil, are widely used for MR imaging in the very-high RF band (VHF) and are being introduced in commercial 3 T whole-body MR imaging scanners. The birdcage coil has multiple conductor rungs which are arranged around an examination region of the MR imaging scanner, extending parallel to a main magnetic field direction. The coils require individual RF power supply to provide acceptable clinical image quality due to individual dielectric loading by the subject of interest.
A degenerated birdcage coil consists of individual loops formed by rungs and ring members. Ring members and rungs are orthogonally arranged and naturally decoupled1). RF currents flowing in these loops can be controlled but mutual coupling may lead to high intra-channel RF power consumption, especially with an increasing number of channels. 1) cf. Hayes C, Edelstein W, Schenck J, Mueller O, Eash M.: An efficient, highly homogeneous radiofrequency coil for whole-body NMR imaging at 1.5 T, J Magn Reson 1985; 63:622-628, or Tropp J.: The theory of the bird-cage resonator, J Magn Reson 1989; 82:51-62
With the advent of low-power multi-channel RF amplifiers, a total number of transmit channels is not limited by cost anymore. High count distributed RF amplifiers can supply body coils with high count of input ports to satisfy the increasing requirements of high field clinical applications.
Document EP 1 279968 A2 describes an example of a bodycoil with individually controlled RF currents in the rungs which enables to vary an RF field pattern of the birdcage coil (“RF shimming”) for the purpose of homogeneity in the presence of dielectric loading of the subject of interest. It is desirable to avoid a high ratio of RF currents in ring members and rungs that may give rise to excessive currents in the ring members and high specific absorption rates (SAR) for the subject of interest which requires special attention. Further, it is also desirable to extend the scope of RF shimming for this type of RF coil.